As a fixing device used in image forming apparatus such as electro-photographic copying machines, printers, there is a fixing device to heat, pressurize and fix toner images by inserting sheet paper between a roller pair comprising a heat roller and a pressure roller or similar belts. In this heating type fixing device, in order to maintain a heat roller at a constant temperature for fixing toner images, a surface temperature of the heat roller is detected by a temperature sensor and a heat source is controlled by turning it ON/OFF according to the detected result.
In recent years, a non-contact type temperature sensor is used, which detects temperature without contacting heating units such as heat rollers, fixing belts like a non-contact type infrared temperature sensors without contacting heating units like heat rollers, fixing belts and so on. Especially, a thermopile infrared temperature sensor is in a structure with a calorific capacity of temperature contact portion of a thin film thermocouple made small and the temperature response is high. As a result, it becomes possible to make the temperature control of the heating units precisely and rapidly.
However, if dirt is adhered on objects for temperature detection, such the non-contact type temperature sensor detects not only the surface temperature of objects but also the temperature of adhered dirt. Therefore, an accurate temperature of object cannot be obtained and erroneous temperatures may be detected. Furthermore, after fixed and cleaned, dirt and dust such as scattered toner, paper dust may be adhered on the surfaces of heating units. Accordingly, when detecting the surface temperatures of heating members of the fixing device by a non-contact type temperature sensor, erroneous temperatures including those of dirt adhered on the heating units may be detected. As a result, temperatures of heating members cannot be controlled accurately and improper fixing may possibly result.
So, in the field of a fixing device to detect surface temperatures of heating members with non-contact type temperature sensors, the development of a fixing device capable of improving fixing efficiency and obtaining high image quality by detecting temperatures of heating members precisely and accurately controlling temperatures of heating members even when there are dirt adhered on heating members is so far desired.